It has become increasingly desirable to incorporate oscillators into integrated circuit design, especially in CMOS and BiCMOS circuits. It is especially desirable, for reasons of cost, simplicity and spatial economy, to render the oscillator as a self-contained circuit having no external circuit components. It is generally desirable to configure such oscillators as components that are frequency stable over a range of variable external parameters, such as temperature, input voltage, and process dependent component parameter. Moreover, it is generally desirable to configure oscillators that have minimal power requirements and footprint, especially when the oscillator is to be incorporated into mobile D.C. power components, such as cellular communications and portable computer equipment.
Notwithstanding the foregoing design objectives, conventional oscillators oftentimes exhibit a frequency error of .+-.30% for each of these external parameters as any one parameter is varied over its typical range of values. For example, such frequency fluctuations are encountered over temperature ranges of about -50.degree. C. to 150.degree. C., voltages of about 2.5 V-10 V, and transistor strength (i.e., current gain ) fluctuations of .+-.30%. The foregoing errors can cumulatively introduce an error of up to about .+-.70%, which is clearly unacceptable for many applications.
Prior art systems have attempted to overcome the foregoing deficiencies, but none have been entirely satisfactory. In U.S. Pat. No. 4,283,690, a low power CMOS oscillator is disclosed for the purpose of maintaining frequency fluctuations within a prescribed range during voltage supply variations. The oscillator is connected to an external circuit through a pin. The system employs an external capacitor that is connected to the inputs of two inverters. One of the inverters is used to drive a p-channel transistor, while the other inverter is connected to an n-channel transistor. Both of the transistors are connected in series and provide an output from a node that is formed between the two transistors. The output of a latch that is connected to the node is coupled to the capacitor to effect capacitor charge and discharge. Disadvantages of this system include the provision of an external capacitor, thereby increasing the cost and spatial requirements of the oscillator, and the use of a pin for connecting to a circuit, thereby reducing the number of available pin connections for other functions.
The problems of temperature and parameter stability in a CMOS oscillator were more recently addressed in U.S. Pat. No. 5,021,750. The disclosed oscillator includes a two-stage differential amplifier and two controlled current sources which are interconnected by a frequency-determining capacitor. The two current sources are connected to the inputs of the differential amplifier through first and second resistors. First and second switching units connected to the differential amplifier and first and second resistors, respectively, complete first and second current paths in response to voltage drops across the first and second resistors to charge the capacitor to first and second respective charge space. Disadvantages of the disclosed CMOS oscillator include the use of a differential amplifier. Such amplifiers typically do not operate efficiently to low values of V.sub.DD. Moreover, the disclosed oscillator provides six current paths as well as a multitude of resistors, thereby requiring considerably more power and die area than is desired in self-powered, portable systems.
Accordingly, there exists a need for compact, self-contained "building block" oscillators for CMOS-type circuits to replace existing, external circuits and which are frequency stable over a wide range of operation parameters to, among other things, provide power management in mobile and other types of communications systems.